(1) Field of the Invention
The present invention relates to a resonator and to an aircraft fitted with the resonator.
The present invention relates to mechanisms seeking to reduce a level of vibration. In particular, the present invention relates to mechanisms seeking to reduce vibrations induced by aeroelastic instabilities that result from coupling between the vibratory modes of the carrier structure of an aircraft and the vibratory modes of a rotor of the aircraft. Such a mechanism can possibly be used to treat excitations of a helicopter tail boom, and in particular to treat the phenomenon known as “tail shake”. The invention provides such a mechanism and an aircraft provided with such a mechanism.
(2) Description of Related Art
Among aircraft, rotorcraft are provided with at least one main rotor connected to an airframe. The main rotor contributes at least in part to providing the aircraft with lift and possibly also propulsion.
Furthermore, engines suitable for driving rotation of the main rotor are arranged in the airframe. Under such circumstances, the airframe includes a mounting structure for the rotor enabling the main rotor to be fastened to the airframe.
Such a mounting structure usually includes a main power transmission gearbox (MGB). The main gearbox is driven by the engines of the aircraft. Furthermore, the main gearbox is provided with a rotor mast for driving the main rotor in rotation.
In addition, the mounting structure includes fastener elements for fastening the main gearbox to a platform of the airframe. For example, a mounting structure of a rotorcraft, referred to by the person skilled in the art as a “pylon”, comprises both a main gearbox and suspension bars.
The airframe and the main rotor are each subjected to forced excitations inherent to the speed of advance of the aircraft. For example, the tail boom of a helicopter airframe may be excited directly by a turbulent air stream coming from the main rotor.
In addition to forced excitations, another vibratory phenomenon can give rise to vibratory problems on an aircraft.
Such a problem lies in the attenuation of vibratory phenomena induced by the aeroelastic instabilities to which an aircraft is subjected in flight. For example, such aeroelastic instabilities may result from the coupling between the vibratory modes of the carrier structure that is caused by aerodynamic effects due to the stream of air moving around said carrier structure, and in particular a structure of the fixed wing or rotary wing type. Such instabilities are known to the person skilled in the art in particular under the general term “flutter”.
Other aeroelastic instabilities correspond by way of example to instabilities known as “whirl flutter”, designating an instability that appears for rotating systems placed in an air stream. By way of example, such instability can result from coupling between the vibratory modes of a bladed rotor and the vibratory modes of the carrier structure supporting the rotor.
These phenomena of “flutter” and of “whirl flutter” are characterized by limit cycle vibration or by diverging vibration that can lead to mechanical parts or structural elements breaking.
These instabilities are generally avoided by suitably selecting mode characteristics for the rotor and/or for the carrier structure. An instability appearing on an existing aircraft thus requires structural modifications to the rotor and/or to the carrier structure, which modifications may be considerable.
Rotorcraft are structured in general terms to mitigate the consequences of such vibration.
Specifically, the rotor or the airframe may be fitted with antivibration systems, sometimes referred to as “resonators”, that serve to minimize the level of vibration.
The function of a resonator is to generate vibration locally to produce effects that counter the unwanted vibration of a carrier structure.
Thus, a resonator conventionally comprises a member referred to as a “seismic mass” or a “moving mass” connected to a support via movement means. The movement means provide the seismic mass with a degree of freedom to move. The movement of the seismic mass is then coupled to the movement of the carrier structure in order to counter the excitation exerted on the carrier structure.
In particular, coupling between the movement of the seismic mass and the movement of the carrier structure can be achieved by using a passive resonator with preponderant stiffness. The movement means of such a resonator then present stiffness that is preponderant relative to other characteristics, and in particular relative to the damping generated by the movement means. The damping is considered as being negligible in the context of a passive resonator with predominant stiffness.
A known passive resonator with preponderant stiffness then includes a seismic mass connected to a carrier structure via a blade type flexible connection.
When the carrier structure is excited in a direction substantially orthogonal to the blade, the seismic mass performs rocking movement.
The effect of the passive resonator with preponderant stiffness is to “smother” the vibration that is giving rise to the movement of the carrier structure by creating antiresonance at a given antiresonance frequency setting. In contrast, a passive resonator with preponderant stiffness generates two new modes of vibration having two respective resonant frequencies that are situated on either side of the antiresonance frequency. The frequency range extending between these two modes of vibration remains relatively narrow.
Under certain circumstances, the two modes of vibration created by the resonator are not troublesome, insofar as the two resonant frequencies that are created differ from the given antiresonance frequency that is to be treated.
Document FR 2 961 570 proposes a semi-active resonator enabling a seismic mass to be moved manually relative to a blade in order to enable the resonator to treat vibratory phenomena that takes place at various different frequencies.
Another known resonator is a damped resonator. The movement means of such a damped resonator then present stiffness that is not preponderant relative to the damping generated by the movement means.
Consequently, the movement means present stiffness and a damping coefficient that are both non-negligible. For example, the movement means may comprise a first member presenting non-negligible stiffness and a second member presenting a non-negligible damping coefficient. By way of example, the first member may be in the form of a deformable member. The second member may be obtained from electromagnetic means having an electric coil and a permanent magnet.
When the carrier structure vibrates, the damped resonator then vibrates in phase quadrature relative to the carrier structure for treatment and thus opposes the movement of the carrier structure.
The mass per se of the seismic mass is selected as a function of the mass of the carrier structure and of the damping to be provided.
The movement means may in particular be adjusted so as to provide optimum damping that makes it possible to minimize the response of the carrier structure over a frequency band.
If the damping provided is small, then the damped resonator behaves like a resonator with preponderant stiffness by causing two resonant peaks to appear on either side of the treated antiresonance frequency. Conversely, if the damping provided is large, then the damped resonator has little impact on the response of the structure. In contrast, when the damping coefficient of the movement means is suitably chosen, the amplitude of the treated vibration at the frequency setting of the resonator is reduced, but without that creating two resonance peaks. Adjustment of the damped resonator thus seeks to achieve a compromise between the desired attenuation of the vibration for treatment at a particular frequency and the potential presence of two resonance peaks produced on either side of that particular frequency.
The performance of a damped resonator is proportional to the mass of the seismic mass. In the event of a modification to the carrier structure of an aircraft, it can nevertheless be difficult to adapt a damped resonator.
In addition, such a damped resonator is used for filtering vibration that occurs substantially at a single given frequency.
Unfortunately, certain aircraft may have a main rotor that rotates at a speed setpoint that can vary during a flight. This situation makes the means for compensating the resulting aerodynamic instabilities more complicated, since for such aircraft it is necessary to attenuate the “broadband” vibration generated by the rotation of the main rotor. An ordinary damped resonator can thus be less effective in this context.
Another type of system comprises an active resonator having an actuator. Such a system consists in controlling the movement of the seismic mass by means of an actuator as a function of the position of the seismic mass.
By way of example, such a resonator is illustrated by Document FR 2 784 350.
That resonator is advantageous. Nevertheless, the damping provided is limited because of its intrinsic instability. A resonator is usually said to be “conditionally stable” insofar as the resonator is stable up to a certain level of excitation. If the resonator is adjusted to counter the vibration exerted on a structure by using an actuator and the dynamic amplification of the seismic mass for generating a force is large, then the operation of the resonator can become unstable and uncontrollable. Under such circumstances, the force exerted by the actuator is limited in order to remain within the stability range of the resonator.
Another active resonator includes a sensor measuring the vibration of the carrier structure fitted with the resonator. The actuator is then controlled as a function of the measured vibration.
Documents U.S. Pat. No. 5,620,068, FR 2 770 825, and U.S. Pat. No. 5,853,144 are also known.
Document U.S. Pat. No. 5,620,068 describes an actuator used for exciting a mass-spring system.
Document FR 2 770 825 describes a system having a resonator, at least one sensor measuring a parameter representative of vibration present in a cabin, and a control unit connected to the resonator and to the sensor.
Document U.S. Pat. No. 5,853,144 describes a helicopter having means for causing control devices to oscillate at a frequency corresponding to an excitation frequency.
Document EP 2 845 799 is also known.
Documents US 2009/020381 and EP 2 039 957 are remote from the field of the invention.